On most prior art exercise machines, when exercising the torso muscles such as the abdominal and lower back muscles, the user engages an upper body engaging member of the machine and exerts back and forth force thereagainst so that the spine of the user partially rotates around several vertebrae. During these back and forth movements, the axis of rotation of the user moves vertically along the spine.
Heretofore, most prior art exercise machines for exercising the abdominal and lower back muscles have been constructed to include a body engaging member moveable in a back and forth semi-circular path of travel which is always the same distance from the rotational axis. For example, in commonly assigned U.S. Pat. Nos. 4,500,089 and 4,387,893, the body engaging member is mounted on the outer end of a user actuated lever. The inner end of the user actuated lever is pivotally connected to the machine frame so that the body engaging member is moved back and forth along a semi-circular path of travel. During the back and forth exercise movements, the lever rotates about a fixed axis of rotation. The maximum exercise efficiency is not obtained because the actual axis of rotation of the body moves vertically along the spine during the back and forth movements while the semi-circular movement of the body engaging member does not compensate for the movement of the rotational axis along the vertebrae of the spine. This prior art type of exercise machine causes rotation around the hips instead of the desired rotational movement along the spine.
Additionally, in most prior art exercise machines such as disclosed in the aforementioned prior art patents, no means is provided for limiting the range of the body engaging member during the exercise. At times, it is desirable to vary the starting position of the body engaging member and thereby reduce its range of movement. For example, a back injury may necessitate adjusting the exercise machine so that instead of exercising the back with the full range of movement from a forwardly bent position to a rearwardly bent position, only a limited range of back movement is provided to prevent further injury.